Since optical communication is a technique that could play a central role in future broadband communications, the optical components used in optical communication systems are required to be higher in performance, smaller in size, and lower in price for widespread use of the optical communication. Optical communication devices using photonic crystals are one of the leading candidates for the next-generation optical communication components that satisfy the above-described requirements. Some of the optical communication devices have already been put into practical use, and an example is a photonic crystal fiber for polarized light dispersion compensation. Furthermore, recent efforts have had a practical goal of developing optical multiplexers/demultiplexers and other devices that can be used in wavelength division multiplexing (WDM) communication.
A photonic crystal is a dielectric object having a period structure. Usually, the period structure is created by providing the dielectric body with a periodic arrangement of modified refractive index areas, i.e. the areas whose refractive index differs from that of the body. Within the crystal, the period structure creates a band structure with respect to the energy of light and thereby produces an energy region in which the light cannot be propagated. Such an energy region is called the “photonic band gap (PBG)”.
Providing an appropriate defect in the photonic crystal creates a specific energy level (“defect level”) within the PBQ and only a ray of light having a wavelength corresponding to the defect level is allowed to be present in the vicinity of the defect. A defect created in a point pattern can function as an optical resonator that resonates with light having a specific wavelength, and a linear defect enables the crystal to be used as a waveguide.
As an example of the above-described technique, Patent Document 1 discloses a two-dimensional photonic crystal having a body (or slab) provided with a periodic arrangement of modified refractive index areas, in which a linear defect of the periodic arrangement is created to form a waveguide and a point-like defect is created adjacent to the waveguide. This two-dimensional photonic crystal functions as the following two devices: a demultiplexer for extracting a ray of light whose wavelength equals the resonance wavelength of the resonator from rays of light having various wavelengths and propagated through the waveguide and for sending the extracted light to the outside; and a multiplexer for introducing the same light from the outside into the waveguide.
Many two-dimensional photonic crystals including the one described in Patent Document 1 are designed so that the PBG becomes effective for either a TE-polarized light, in which the electric field oscillates in the direction parallel to the body, or a TM-polarized light, in which the magnetic field oscillates in the direction parallel to the body. In this case, if the PBG is not created for the other polarized light or created in an energy region which differs from that of the PBG of the given polarized light, the TE-polarized light and the TM-polarized light cannot be simultaneously used in the same frequency, wavelength and energy.
For example, if it is further made possible to use the TE-polarized light and the TM-polarized light independently with respect to the same frequency in the WDM, the multiplexing number can be doubled in comparison with the case without using the polarized light independently (polarized light multiplexing). However, individual use of both the TM-polarized light and the TM-polarized light in the same frequency is impossible in an optical multiplexer/demultiplexer using the conventional two-dimensional photonic crystal, so that polarized light multiplexing is difficult to perform.
Taking the above problem into account, studies have been conducted on a new design of a two-dimensional photonic crystal having a PBG for each of the TE-polarized light and the TM-polarized light in which the two PBGs have a common band. This common band is called the “absolute photonic band gap (absolute PBG)” hereinafter. For example, a two-dimensional photonic crystal disclosed in Patent Document 2 has an absolute PBG created by periodically arranging triangular (or triangle-pole-shaped) holes in a triangular lattice pattern in a slab-shaped body. In this two-dimensional photonic crystal described in Patent Document 2, light whose frequency is within the absolute PBG can be used as the TE-polarized light and the TM-polarized light independently.    [Patent Document 1] Unexamined Japanese Patent Publication No. 2001-272555 ([0023]-[0027.], [0032], FIGS. 1, and 5-6)    [Patent Document 2] Unexamined Japanese Patent Publication No. 2004-294517 ([0021]-[0022.], [0041]-[0043], FIGS. 1, and 14-17)